This invention relates to polymer blend compositions and the process for preparing the same. In particular, the invention relates to novel blends of cationic polymers and anionic polymers which exhibit properties that make them useful as coatings for various substrates.
Ink jet printing is widely used to print on a variety of substrates (including paper, textiles, and plastics). These substrates are often coated with a material that enhances their receptivity for the ink jet ink. In the case of aqueous dye-based inks, which comprise the majority of inks currently used in ink jet printing, the property of dye fixation is particularly important. Most aqueous ink jet inks are based on dyes rather than pigments. To obtain sharp prints with high color density, the dye molecules must be immobilized almost immediately upon contact of the ink with the substrate. Penetration of the dyes into the substrate will result in reduced color density, while lateral migration of the dye molecules will cause indistinctness in the image formed.
The dyes that are commonly employed in aqueous ink jet inks are anionic in nature, containing sulfonic acid groups. Accordingly, dye fixation is generally accomplished by incorporating cationic polymers into the ink jet receptive coatings. These cationic polymers employ salt formation mechanisms to fix the dyes. The most widely used cationic dye fixative in ink jet receptive coatings is poly(diallyldimethylammonium chloride), although other water-soluble cationic polymers are known in the art. For example, U.S. Pat. No. 6,010,790 teaches the use of poly(vinylbenzylquaternary ammonium salts). Other examples of water-soluble cationic polymers are cationic starch, cationic polyvinyl alcohol, guanidine-formaldehyde resins, epichlorohydrin-polyamine condensates, and water-soluble cationic acrylic resins.
However, problems exist with the use of soluble or water-borne cationic polymers. For example, unless these polymers are cross-linked in some way, their presence often has a detrimental effect on the water resistance and chemical resistance of ink jet prints and other coatings. Also, as these cationic polymers are relatively expensive, their use in traditional packaging applications is limited. Furthermore, in many applications it is desirable to match the gloss of the coating to the appearance of the substrate. However, due to the glossy nature of most cationic polymers, it is often necessary to use gloss-modifying pigments to adjust the appearance of the finished coating. Also, the chemical nature of cationic polymers limits the materials available to modify the physical properties of the cationic-based coatings, in that many of the additives commonly used with anionic-based coatings would form precipitants or gels if employed with cationic-based coatings. Furthermore, as coatings produced with cationic polymers are normally transparent in nature, the color development of inks may be adversely effected by the color of the underlying substrate. Finally, cationic-based coatings are usually much slower to dry than anionic coatings. As most machinery used in coatings (e.g., flexographic printing presses, gravure printing presses, rod-coater, etc.) are designed for use with faster-drying anionic systems, the applicability of cationic-based coatings systems are limited.
Anionic polymers are normally added to a coating formulation in order to improve the chemical resistance, water resistance, and/or rub resistance of the coating. However, anionic polymers are not commonly employed in ink jet receptive coatings because, as most ink jet inks are composed of anionic dyes, an anionic ink jet receptive coating would not adequately fixate the dyes to the coated substrate.
It would be desirable to be able to produce a stable blend of cationic polymers and anionic polymers for use in various coating applications.
Therefore, an object of this invention is to disclose stable polymer blend compositions comprising mixtures of cationic polymer compositions and anionic polymer compositions.
Another object of this invention is to disclose polymer blend compositions which exhibit properties that allow them to be useful as coating formulations.
A further object of this invention is to disclose polymer blend compositions which exhibit properties that allow them to be useful in producing ink jet ink printing coating formulations.
The present invention achieves these objects and others via the production of polymer blend compositions that are stable mixtures of cationic polymer compositions and anionic polymer compositions. These novel blends minimize many of the problems involved in using either a cationic polymer composition or an anionic polymer composition in various coatings.
Skilled artisans would anticipate that the blending of cationic polymer compositions with anionic polymer compositions would not produce a stable polymer blend, as the negatively-charged particles and the positively-charged particles would tend to aggregate together to form a gel or to produce coagulum in the polymer solution. It was, therefore, unexpected that stable polymer blend compositions which exhibited no precipitate formation or gelling could be produced via the present method.
By way of explanation, but without limitation, this result may be caused by the employment of the particular cationic polymer compositions and anionic polymer compositions of the present invention. The free radical polymerization reaction utilized herein results in the production of cationic polymer compositions that contain at least one cationic functionality in the backbone of the polymer. The chemical characteristics of these cationic polymer compositions, coupled with the chemical characteristics and relatively low acid numbers of the anionic polymer compositions, may serve to retard the susceptibility of the respective polymers to react chemically.
Irrespective of the exact chemical mechanism, the ability to create stable water-borne polymer blend compositions which comprise a mixture of cationic polymers and anionic polymers allows the practitioner to produce compositions having chemical characteristics not easily obtained through the use of either cationic polymers or anionic polymers alone. For example, the employment of polymer blend compositions allows one to match the gloss of coatings to substrates, thereby improving the appearance of the printed finished product. Also, a stable polymer blend composition permits the utilization of a cationic polymer to modify an anionic coating system to produce desired coating viscosities without the loss of chemical resistance or other advantageous properties. Moreover, the ability to blend cationic polymer compositions with anionic polymer compositions allows one to produce a polymer blend coating compositions with desired drying rates. Finally, the broader versatility exhibited by the polymer blend ink jet receptive coating compositions of the present invention allows their employment in a variety of applications not commercially viable for traditional cationic ink jet receptive coatings.